Antenna for orbiting satellite

ABSTRACT

An orbiting satellite system with an antenna for re-transmitting to the ground images collected by image capture instruments of the satellite, the antennas having more than one elementary radiating antenna each of which has more than one cord regularly distributed in a helix about a generatrix of revolution and equi-amplitude power supply for the various cords where the axis of the various elementary antenna are parallel and aligned in one and the same plane in which they are spaced regularly apart in that plane. The plane of the antennas is intended to align with, when the satellite is in orbit, the direction perpendicular to the direction of the speed vector of the satellite. The antenna also has a phase shifting power supply which enables the antenna array to carry out electronic steering of the elongate beam generated by the elementary array.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to antennas for orbiting satellites.

2. Description of the Related Art

Hitherto, the antennas used by orbiting satellites are either antennasof the omnidirectional type (SPOT, ERS, etc.) or of the steerabledirectional type (LANDSAT, etc.).

In the latter case, the beam is Gaussian and scanning is carried outwith the aid of a pointing mechanism, the antenna itself behaving as acentered parabolic reflector of conventional design.

SUMMARY OF THE INVENTION

One purpose of the invention is to propose an antenna for orbitingsatellite which requires no pointing mechanism, which exhibits a greatergain than omnidirectional antennas and which is compact and inexpensive.

To this end, the invention proposes an antenna for retransmitting to theground images collected by image-capture instruments of an orbitingsatellite, characterized in that it comprises a plurality of elementaryradiating antennas of the type having a plurality of cords regularlydistributed in a helix about one and the same generatrix of revolutionas well as means for the equi-amplitude power supply of the variouscords, in that these various elementary antennas are aligned and in thatthe plane in which these various elementary antennas are distributed isintended, when the satellite is in orbit, to be perpendicular to thedirection of the speed vector of the satellite and in that it alsocomprises means for phase-shifting the power supply to these variouselementary antennas which are able to carry out electronic steering ofthe elongate beam generated by the said elementary antennas.

It will be noted that with such a distribution of elementary antennaswith a shaped pattern, the transmit beam produced is a beam of elliptictype (known as “fan beans” [sic]) which extends in a direction parallelto that of the speed vector of the satellite.

The steering of this beam to a given longitude makes it possible toreach, throughout the time of transit of a satellite, a station locatedat this longitude, and to do so without needing to modify this steeringas the satellite advances.

It is understood that such an antenna structure does not requirecomplicated electronics and allows high transmission bit rates.

This antenna is advantageously supplemented with the following variouscharacteristics taken alone or according to all their possiblecombinations:

the number of elementary radiating elements is equal to or greater thanfive;

the elementary radiating elements are staggered one with respect toanother with a spacing which is chosen so as to avoid the grating lobes;

for a transmission frequency of 8000 MHz, the spacing between twoelementary antennas is of the order of 19 mm;

the phase-shifting means are coded over three to eight bits;

the phase-shifting means are of the ferrite type.

Other characteristics and advantages of the invention will emergefurther from the following description. This description is purelyillustrative and nonlimiting. It should be read in conjunction with theappended drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation illustrating an antenna inaccordance with one embodiment of the invention;

FIG. 2 is a graph on which has been plotted the pattern of an elementaryradiating element of the antenna of FIG. 1;

FIGS. 3 to 6 illustrate various coverage patterns obtained with theantenna of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The antenna illustrated in FIG. 1 comprises a plurality of elementaryradiating elements referenced by 1.

These elementary radiating elements 1 each comprise a plurality ofhelical cords regularly distributed about one and the same generatrix ofrevolution. The generatrix is for example conical or cylindrical. Thesecords are powered in an equi-amplitude manner.

For example, these cords are four in number and define four identicalhelices, staggered by π/2 with respect to one another. These four cordsare advantageously phase-quadrature-powered.

The angular radiating pattern of such an elementary radiating element isof the type illustrated in FIG. 2.

This pattern corresponds to the pattern obtained for an axial height ofradiating element of 0.050 m, a base radius of 0.018 m, and atransmission frequency of 8000 MHz. It is referred to a measurementsphere 10 mm in diameter.

It will be noted that the elementary radiating elements with severalhelical cords have, as will be seen later, the advantage of exhibitinggreater gains at 500 than at 0° and hence of making it possible tocompensate for steering losses.

The elementary radiating elements 1 are distributed in line in a planeperpendicular to the direction of the speed vector.

They are arranged in such a way that their axes are parallel, in one andthe same plane and spaced regularly apart. The spacing between the saidradiating elements 1 is for example 19 mm for a transmission frequencyof 8000 MHz, thereby preventing grating lobes.

More generally, the spacing d of the array is such that

d<λ/(1+sin θ)

where λ is the wavelength of the radiation, and θ the maximum amount ofsteering desired.

The radiating elements 1 are powered via phase-shifters 2 of ferritetype and couplers 3, through a power distributor 6 (in this instance1:5), which is for example of waveguide type.

The phase-shifters 2 are controlled by a unit 4 which is the satelliteon-board computer, to which unit they are linked by control electronics5.

The use of ferrite type phase-shifters has the advantage of making itpossible always to retain the same amount of steering. The consumptionof the control electronics is then limited.

The phase shifts imposed on the various radiating elements 1 make itpossible to produce the desired amounts of steering, up to ±62°.

The choice of a helix structure for the radiating elements 1 makes itpossible to attain a gain at 500 which is 2 dB greater than the gain at0° (excluding the term for compensating for difference in spaceattenuation −62° satellite up with respect to the zenith) and hence tocompensate naturally for the steering losses.

The optimal number of elementary radiating elements will vary from fiveto twelve depending on the requirements of the mission.

The phase-shifters 2 have for example quantization spacings of 22.50 andare coded over 4 bits.

The beams generated by such an antenna are elliptic (major axis of theellipses parallel to the track of the satellite).

FIG. 3 illustrates the coverage obtained with the antenna justdescribed, in the case of a zero phase shift between the variousradiating elements 1.

There is then no steering and the maximum directivity of the antenna is11.55 dB.

Represented in FIG. 4 is the coverage obtained in the case of phaseshifts respectively from one end radiating element 1 to the other of90°, 45°, 0°, −45° and −90°.

The pattern is then steered by +18°. The directivity is 11.52 dB.

Illustrated in FIG. 5 is the coverage obtained in the case of a phaseshift of 180°, 90°, 0°, −90°, −180°, respectively.

The steering is then 320, the directivity 11.49 dB.

Finally, represented in FIG. 6 is the coverage obtained for phase shiftsof 270°, 135°, 0°, −135° and −270°, respectively.

The steering obtained is 48°, the maximum directivity 11.45 dB.

In these various FIGS. 3 to 6, the circles represented by dashed linescorrespond to the circles of visibility at ±60° and ±65°, respectively.

It is noted that, from one pattern to another, the maximum directivityvaries very little (11.54 dB to 11.45 dB).

The directivity obtained at 650 is greater than 9-dB [sic], i.e. a gainof greater than 7.5 dB if losses of 0.5 dB are considered with regard tothe distributors, of 0.5 dB with regard to the phase-shifters, of 0.25dB with regard to the connection facilities and of 0.25 dB with regardto the power supply.

The steerable antenna just described allows considerable bit rates forretransmission to the ground and allows retransmissions ofhigh-resolution images.

The switching of the beam is preferably performed before transit, so asto avoid the problems of phase hopping over the coverage generated.

In the case where the antenna pattern does not compensate for the spaceattenuation, it is possible to envisage changes of transmission speed soas to make best use of the gains of the antenna in areas close to thezenith transit.

The steerable antenna just described has the advantage of beinginexpensive and especially of small proportions. The proportions of theradiating part are 90 mm long, 5 mm wide and 50 mm high.

Again advantageously, the antenna comprises several in-line antennas ofthe type just described, and switching means making it possible toswitch over from one in-line antenna to another as a function of themovements of the satellite, and in particular of its roll movements.

As a variant, the antenna comprises motorization means which make itpossible to modify the orientation of the line or lines of elementaryradiating elements so as to compensate for the potential movements ofthe satellite, in particular its roll movements.

What is claimed is:
 1. A system comprising: an orbiting satellite and anantenna for retransmitting to the ground images collected byimage-capture instruments of said satellite, wherein said antennacomprises; a plurality of elementary radiating antennas (1) of the typehaving a plurality of cords regularly distributed in a helix about oneand the same generatrix of revolution as well as means for theequi-amplitude power supply of said cords, in that the axes of saidplurality of elementary antennas are parallel to each other and arealigned in one and the same plane in which they are spaced regularlyapart in that said plane in which said elementary antennas aredistributed is intended, when said satellite is in orbit, to beperpendicular to the direction of the speed vector of said satellite andin that said antenna also comprises means (2) for phase-shifting thepower supply to said elementary antennas which are able to carry outelectronic steering of the elongate beam generated by said elementaryantennas.
 2. The system according to claim 1, wherein the number ofelementary radiating elements (1) is equal to or greater than five. 3.The system according to claim 2, wherein the elementary radiatingelements (1) are staggered one with respect to another with a spacingwhich is chosen so as to avoid the grating lobes.
 4. The systemaccording to claim 3, wherein the phase-shifting means are coded overthree to eight bits.
 5. The system according to claim 4, wherein thephase-shifting means (2) are of the ferrite type.
 6. The systemaccording to claim 3, wherein the phase-shifting means (2) are of theferrite type.
 7. The system according to claim 3, wherein, for atransmission frequency of 8000 MHz, the spacing between two elementaryradiating elements is of the order of 19 mm.
 8. The system according toclaim 7, wherein the phase-shifting means are coded over three to eightbits.
 9. The system according to claim 8, wherein the phase-shiftingmeans (2) are of the ferrite type.
 10. The system according to claim 7,wherein the phase-shifting means (2) are of the ferrite type.
 11. Thesystem according to claim 2, wherein the phase-shifting means are codedover three to eight bits.
 12. The system according to claim 11, whereinthe phase-shifting means (2) are of the ferrite type.
 13. The systemaccording to claim 2, wherein the phase-shifting means (2) are of theferrite type.
 14. The system according to claim 1, wherein theelementary radiating elements (1) are staggered one with respect toanother with a spacing which is chosen so as to avoid the grating lobes.15. The system according to claim 14, wherein, for a transmissionfrequency of 8000 MHz, the spacing between two elementary radiatingelements is of the order of 19 mm.
 16. The system according to claim 15,wherein the phase-shifting means are coded over three to eight bits. 17.The system according to claim 16, wherein the phase-shifting means (2)are of the ferrite type.
 18. The system according to claim 15, whereinthe phase-shifting means (2) are of the ferrite type.
 19. The systemaccording to claim 14, wherein the phase-shifting means are coded overthree to eight bits.
 20. The system according to claim 19, wherein thephase-shifting means (2) are of the ferrite type.
 21. The systemaccording to claim 14, wherein the phase-shifting means (2) are of theferrite type.
 22. The system according to claim 1, wherein thephase-shifting means are coded over three to eight bits.
 23. The systemaccording to claim 22, wherein the phase-shifting means (2) are of theferrite type.
 24. The system according to claim 1, wherein thephase-shifting means (2) are of the ferrite type.
 25. The systemaccording to claim 1, wherein the antenna comprises at least two in-lineantennas and switching means for switching from one in-line antenna toanother as a function of movement of the satellite.
 26. The systemaccording to claim 25, wherein the satellite movement is roll.
 27. Thesystem according to claim 1, wherein the antenna comprises motorizationmeans for modifying the orientation of the line or lines of elementaryradiating element so as to compensate for the potential movements of thesatellite.
 28. The system according to claim 27, wherein the satellitemovement is roll.
 29. An orbiting satellite comprising image-captureinstruments and an antenna for retransmitting to the ground imagescollected by said image-capture instruments, said antenna comprising aplurality of elementary radiating antennas (1) of the type having aplurality of cords regularly distributed in a helix about one and thesame generatrix of revolution as well as means for the equi-amplitudepower supply of the various cords, the axes of said plurality ofelementary antennas being aligned in a plane, and due to this planeconfiguration, emitting together an elongate beam, the plane in whichthe axes of said plurality of elementary antennas are distributed beingperpendicular to the direction of the speed vector of the satellite, theantenna comprising means (2) for phase-shifting the power supply to saidplurality of elementary antennas with phase shifts regularly distributedfrom one end of the antenna to the other in order to steer the elongatebeam generated by the said elementary antennas.